Unit 1 Lecture 6 1

Different Conversion/Transmission Schemes 2 Before we discuss various line coding schemes, let us first have an idea of different data conversion schemes.

Digital to digital conversion Line Coding Block Coding Scrambling Analog to Digital Conversion PAM PCM – Nyquist Theorem Digital To Analog Conversion ASK, FSK, PSK & QAM – Constellation Analog to Analog Conversion AM, FM &PM 3 Different Techniques Used in Data Transmission/Conversion

4.4 Line Coding Schemes Line Coding Schemes We can roughly divide line coding schemes into five broad categories, as shown in Figure. There are several schemes in each category.

4.5 Figure: Line coding scheme

Digital to Digital Encoding 6

Types of Digital to Digital Encoding 7

Unipolar Encoding 8 Unipolar encoding uses only one voltage level or one polarity This polarity is assigned to one of the two binary states usually the 1 & other state is usually 0 The average amplitude of a unipolar encoded signal is nonzero. This creates a DC component with zero frequency. That means it can travel only through media which can handle DC component. This is almost an obsolete method today.

Types of Polar Encoding 9

Types of Bipolar Encoding 10

Polar encoding Polar encoding uses two voltage levels (positive and negative). Polar encoding uses two voltage levels (positive and negative). By using both levels, the average voltage level on the line is reduced & the DC component problem of unipolar encoding is alleviated. By using both levels, the average voltage level on the line is reduced & the DC component problem of unipolar encoding is alleviated. There are three most popular variations of polar coding There are three most popular variations of polar coding 1.Non Return to Zero (NRZ) 2.Return to Zero (RZ) & 3.Biphase Encoding 11

Types of Polar Encoding 12

Non Return Zero (NRZ) In NRZ encoding, the level of signal is always either positive or negative. The two most popular methods of NRZ transmission are: 1.NRZ-L (NRZ Level) 2.NRZ-I (NRZ Inversion) NRZ-L : in NRZ-L encoding the type of the signal depends on the type of bit it represents. A positive voltage usually means the bit is 0 or negative voltage means bit is 1 or vice versa. Thus the level of voltage depends on the level of the bit. 13

NRZ-I: it is a method, in which the inversion of the voltage level represents a 1 bit. It is a transition between a positive & a negative voltage, not the voltages themselves that represent a 1 bit. A 0 bit is represented by no change. Out of two methods the NRZ-I is superior to NRZ-L due to the synchronization provided by the signal change each time a 1 bit is encountered. The existence of 1s in the data stream allows the reciever to resynchronize. 14

In NRZ-L the level of the signal is dependent upon the state of the bit. Tips 15 In NRZ-I the signal is inverted if a 1 is encountered. NRZ-I is used in USB, Compact CD & Fast Ethernet

NRZ-L NRZ-I Binary Data Figure NRZ-L and NRZ-I encoding time Transition because next bit is 1. Amplitude

Figure NRZ-L and NRZ-I encoding 17 VARIATION 2 : WHEN 0 IS HIGH VOLTAGE

4.18 Figure: Polar schemes (NRZ-L and NRZ-I)

A system is using NRZ-I to transfer 10-Mbps data. What are the average signal rate and minimum bandwidth? Example Solution The average signal rate in NRZ-I coding is S = N/2 = 500 kbaud. The minimum bandwidth for this average baud rate is B min = S = 500 kHz

Return to Zero (RZ) RZ (return-to-zero) refers to a form of digital data transmission in which the binary low and high states, represented by numerals 0 and 1, are transmitted by voltage pulses having certain characteristics. The signal state is determined by the voltage during the first half of each data viz 0 or 1. The signal returns to a resting state (called zero) during the second half of each bit. The resting state is usually zero volts In RZ transmission, the signal changes not between bits but during each bit. To summarize, RZ uses three values Positive, negative & zero 20

4.21 Figure: Polar schemes (RZ) RZ scheme uses self clocking

A good encoded digital signal must contain a provision for synchronization. Tip 22

Biphase Encoding The best solution to the problem of synchronization is biphase encoding. In this method the signal signal changes at the middle of the bit but does not return to zero. Instead it continues to the opposite pole. As in RZ these mid intervals transitions allow for synchronization. It has two variations 1.Manchester encoding 2.Differential Manchester Encoding 23

Manchester Encoding In telecommunication and data storage, Manchester coding (also known as phase encoding, or PE) is a line code in which the encoding of each data bit has at least one transition and occupies the same time. It therefore has no DC component, and is self-clocking. 24

Manchester Encoding Manchester encoding uses the inversion at the middle of each bit interval for both synchronization and bit representation. A negative-to-positive transition represents binary 1 and a positive-to-negative transition represents binary 0. By using a single transition for a dual purpose, Manchester encoding achieves the same level of synchronization as RZ but with only two levels of amplitude. 25

Figure Manchester encoding 26

In Manchester encoding, the transition at the middle of the bit is used for both synchronization and bit representation. Tip 27

Differential Manchester Encoding Differential Manchester encoding is a line code in which data and clock signals are combined to form a single 2-level self-synchronizing data stream. It is a differential encoding, using the presence or absence of transitions to indicate logical value.clock signals It is not necessary to know the polarity of the sent signal since the information is not kept in the actual values of the voltage but in their change: in other words it does not matter whether a logical 1 or 0 is received, but only whether the polarity is the same or different from the previous value; this makes synchronization easier. 28

Differential Manchester Encoding In differential Manchester, the inversion at the middle of the bit interval is used for synchronization, but the presence or absence of an additional transition at the beginning of the interval is used to identify the bit. A transition means binary 0 and no transition means binary 1. Differential Manchester requires two signal changes to represent binary 0 but only one to represent binary 1. 29

Figure Differential Manchester encoding 30

In differential Manchester encoding, the transition at the middle of the bit is used only for synchronization. The bit representation is defined by the inversion or noninversion at the beginning of the bit. Tip 31

4.32 Figure : A Combined Look to Manchester & Differential Manchester Encoding The minimum bandwidth of Manchester & Differential Manchester is 2 times that of token bus & Ethernet

Various Encoding Schemes

More examples of encoding schemes 34

T b T b A 0 A/2 0 -A/2 A/2 0 -A/2 A 0 -A A/2 0 -A/2 Binary Data Unipolar Unipolar NRZ Polar RZ Polar NRZ Bipolar NRZ (AMI) Split phase Manchester Figure : Various line codes for data stream t t t t t t

T b A/2 0 -A/2 -3A/2 3A/2 Polar Quaternar y NRZ

Practical Usage of these coding Manchester code has been specified for the IEEE (Ethernet) standard for baseband coaxial cable and twisted-pair bus LANs. Manchester encoding is also used in IEEE (token bus)IEEE Differential Manchester is specified in the IEEE standard for token ring LANs, and is used for many other applications, including magnetic and optical storage.IEEE